The following notes were taken primarily from Physics for IB by Chris Hamper and Physics Course Companion by Tim Kirk

Topic E: Astrophysics

Are they real?

E.1.1Outline the general structure of the solar system.Students should know that the planets orbit the Sun in

ellipses and moons orbit planets. (Details of Kepler’s laws are not required.) Students should also know the names of the planets, their approximate comparative sizes and comparative distances from the Sun, the nature of comets, and the nature and position of the asteroid belt.

http://solarsystem.nasa.gov/planets

Tons of interesting stuff about our solar system

Aug. 2006 the International Astronomical Union declared the official definition of a planet:A “planet” is a celestial body that:

a) is in orbit around the Sunb) has sufficient mass for its self-gravity to overcome

rigid body forces so that is assumes a hydrostatic equilibrium (nearly round) shape

c) has cleared the neighborhood around its orbit.

8 Planets – name them….Elliptical orbits – to have a circular orbit an object must

have a very specific velocity. Any variations create and elliptical or hyperbolic shape

MoonsPeriod - 27.3 daysWhich planets have them????

Our Solar System

Phases of the moon

Asteroids belt between Mars and Jupitersize – dust to hundreds kilometeres.

CometsSimilar to asteroids but made up of loose particles of ice and

rock.Tail is blown off by solar winds and melted by radiation.Some orbit, others only pass the sun once

Planetoids???Pluto….

Our Solar System

Relative size video.

http://www.wimp.com/starsize/

E.1.3Define the light year.

Light year (ly) – the distance that a beam of light will travel in one year. How far is that? (3 x 108m/s = c)Used to measure distances outside of our solar system

Other important units.Astronomical unit (AU) – the average distance between the

Sun and Earth1AU = 1.5 x 1011mUsed to measure distances inside our solar system

Parsec (pc) – 1parsec = 3.26 lyDefined by making a triangle between the Earth, the Sun and a

distant object. If the angle at the distant object is 1 arcsec then it would be 1 parsec away. (more later)

Super easy

E.1.2Distinguish between a stellar cluster and a constellationE.1.4Compare the relative distances between stars within a

galaxy and between galaxies, in terms of order of magnitude.

Stars are not evenly distributed.Stellar cluster – small groups of stars that gravitationally

interact with one another. Physically close to each otherClosest star, besides the sun is Proxima Centauri - 4.25ly

Distribution of stars

Galaxy – a very large number of stars bound together by gravityTrillions of stars103 – 105 light years acrossEach star is approx. 1 ly apartAndromeda is about 2.5x106ly away

Galaxy cluster – small group of galaxies that gravitationally interact with one another There are about 20 other galaxies we are clustered with.

Supercluster – bigger than a cluster

Distribution of stars

Virgo Cluster

Andromeda

The simplest explanation is that if all the gas is made into stars before the gas has time to form a disk,

then you get an elliptical galaxy. if the gas has time to stabalize into a disk before it is all used up,

then you get a spiral galaxy.Or perhaps some of the elliptical galaxies are made from merging

of other types of galaxies.Observations of distant galaxies indicates that spiral galaxies were

more common in the past than they are today.So maybe yesterday's spirals are todays ellipticals.

This is an active research area. One problem is that if most of the mass in galaxies is unaccounted for, we have a hard time understanding the dynamics of galaxy formation.

How did galaxies get that way?

Constellations – groups of stars that are “linked” visually Ancient civilizations played “connect the dots” Located in the same general direction from EarthNot necessarily close to each other88 totalDifferent ones are visible at different times during the year.

E.1.5Describe the apparent motion of the stars/constellations

over a period of a night and over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth.

This is the basic background for stellar parallax. Other observations, for example, seasons and the motion of planets, are not expected

Why do the stars move through the night sky?Because the rotation of the EarthIt also matters where you are located on Earth Ex. North Polehttp://www.yorku.ca/ns1745b/figs-ch1.html

This rotation takes 23h and 56min every time.The effect is that it seams that the stars position at 12:00

changes each night. This means that the Earth rotates 360º in 23h and 56min.

Which means 4min it will rotate 1º.Which means it only takes 360 DAYS for the constellations

to make one compete rotation.

The Sun doesn’t make the same path through sky every day.

For us, the summers are high in the sky, winters are low on the horizon.

This is because the axis of rotation for the Earth and the axis in which we orbit around the sun aren’t the same angle.

Sun rise

The Earth is not a perfect sphere. This means that depending on it’s location in it’s orbit, it

will feel more or less pull from the SunThis pulls on the Earths axis of rotation and makes it

wooble.This is technically called presession. This means that the “North Star” won’t always be the

north star.Period – 26,000 years

Precession

The word planet comes from the Greek word for wanderer.Planets will shift back and forth in the night sky relative to

the constellation background.Apparent east/west motion comes from the Earth’s orbit

around the sun.See Diagram on board

Apparent north/south motion come from the other planet’s orbital plane being at a different angle from ours.

Other plant’s movement